1. Field of the Invention
The invention relates to a method and device for frequency conversion, particularly for frequency doubling and particularly of continuous fixed frequency laser radiation, of semi-monolithic design according to the preambles of claims 1 and 8.
Numerous methods and devices are already known which describe a frequency conversion of continuous laser radiation by means of non-linear crystals, particularly the generation of the 2nd harmonic wave (frequency doubling) from coherent fundamental wave radiation, with the objective of increasing the conversion efficiency.
2. Description of the Related Art
The "classical arrangement" for the frequency conversion of laser radiation as described moreover in the publications of M. Brieger et al.: "Enhancement of Single Frequency SHG in a Passive Ring Resonator" Opt. Commun. 38 (1981) p. 423; C. S. Adams et al.: "Tunable narrow linewidth ultraviolet light generation . . . ", Opt Commun. 90 (1992) p. 89; S. Bourzeix et al.: "Efficient frequency doubling of a continuous wave . . . ", Opt Commun. 99 (1993) p. 89, consists of a resonator in the form of a double Z composed of four mirrors, at least two of which possess a radius of curvature, and a non-linear crystal. A first mirror is mounted on a piezo-element and serves to tune the resonator length to resonance with the incident light wave. The part of the incoming wave reflected by a third mirror is recorded by a detector. A control signal for active resonator stabilisation can be gained from this by the usual methods (Hansch-Couillaud, Pound-Drever). The mirror distances, radii of curvature and coatings as well as the crystal itself are configured such that
a) the resonator is optically stable, PA1 b) between the two mirrors positioned in the location of the non-linear crystal a beam-waistline is formed, the size of which is optimal for an efficient conversion, PA1 c) the astigmatism of the second beam-waistline caused by the curved mirrors between two further mirrors (third and fourth mirrors) is compensated for by the Brewster cut crystal, PA1 d) three of the mirrors possess as high a reflectivity as possible for the fundamental wave, PA1 e) one of the mirrors possesses as high a transmission as possible for the harmonic generated, PA1 f) the degree of reflection of the coupling input mirror is such that the resonance step-up of the fundamental wave is a high as possible, which is the case where the impedance matching R=1-V (R: degree of reflection, V: passive resonator losses) PA1 g) The precondition for phase matching is fulfilled for the non-linear crystal. PA1 a) they are of essentially greater mechanical stability and thus less susceptible to external disturbances. PA1 b) they have less losses because of a lower number of boundary faces in the resonator. PA1 c) Expensive precision optical components can be done without. PA1 1. The arrangement of the components results in an extremely compact design of the conversion unit and can thus be integrated in a fixed frequency laser without appreciably enlarging the design of the system. PA1 2. The form of the optically non-linear crystal exclusively exhibits flat faces and can thus be produced simply and economically in large quantities compared with the competitive methods. PA1 3. The angle of incidence of the fundamental wave on the refracting faces of the crystal as well as the angle of incidence on the curved mirror are small and astigmatism can thus be neglected. PA1 4. The diminishment of passive losses resulting from the reduced number of constructional components implements an essential improvement of the conversion efficiency from the fundamental wave to the harmonic wave. PA1 5. The stability resulting from compactness and the reduced number of components as well as the piezo-adjustment of the resonator length permit an industrially relevant application of continuous UV laser radiation with fixed wavelengths below 300 nm for the first time. PA1 6. As a result of the shorter length of the resonator as compared to the classical design, the free spectral region of the resonator becomes significantly larger. In this way, the acceptance width of the resonator (using the same quality of resonator) is enhanced in terms of the frequency band width of the incident radiation. Thus, laser radiation sources of poorer quality can also be converted with this arrangement.
Conversion efficiencies between 10% and 30% are typically attained with such arrangements.
Since four adjustable mirror supports are needed in this arrangement, the mechanical effort is relatively high. Since highly reflecting mirrors always exhibit a residual transmission, the passive losses of this arrangement cannot be reduced at will, resulting in an upper limit for the amplification factor of the resonator. All in all, the arrangements are mechanically too unstable and too large to be used as modules for frequency conversion, particularly frequency doubling for fixed frequency lasers in industrially relevant applications.
In other publications, such as in U.S. Pat. Nos. 5,027,361, 5,227,911, 4,731,787, 4,797,896 monolithic or, as described in U.S. Pat. No. 5,007,065, semi-monolithic arrangements are used.
As opposed to discretely assembled resonators, these arrangements have various advantages:
Doubling efficiencies of up to 80% can be achieved with these arrangements. However, production of the crystals forming the monolithic resonators (U.S. Pat. Nos. 5,027,361, 5,227,911, 4,731,787, 4,797,896), is very troublesome (spherically ground faces, special coatings etc.). These crystals are not available on the market and can only be produced in special laboratories. Characteristics of crystals are also partly used which are only met by a few materials such as, for example, the high coefficient for the electro-optical effect of the crystal material lithium metaniobate, which is used to tune the resonator with an electrical voltage (K. Schneider et al.: "1:1-W single frequency 532 nm radiation by second-harmonic generation of a miniature Nd:YAG ring laser", Optics Letters Vol. 21 No. 24, (1996). Restriction to crystals such as these leads to a marked limitation of the wavelength region which can be doubled in this way. In particular, these arrangements cannot be used for generation of the 2nd harmonic in the region below 300 nm.
A semi-monolithic resonator form is described in U.S. Pat. No. 5,007,065, which, together with the laser medium form proposed, was designed as an active laser resonator. In the form described, this resonator cannot be used as a passive resonator for the generation of optical harmonics in an optically non-linear crystal.